After law school, Marshall went on to serve with distinction in the Department of Justice where she received the Antitrust Division's Outstanding Contribution Award for service. Her career in public service continued when she became the Senior Deputy Attorney General for the state of Nevada under Nevada Attorney General Frankie Sue Del Papa.[3]

Marshall was initially elected Treasurer of Nevada in 2006. She won the Democratic nomination with over 65% of the vote and defeated Mark Destefano in the general election by a 47-41 margin. She was elected to a second term on November 2, 2010, against Steve Martin with 48% of the vote, in what was considered a bad electoral year for Democrats overall.

According to Marshall's Treasury website: "A crowning achievement of her term as State Treasurer is the successful sponsorship and passage of Senate Bill 75 during the 2011 Legislative Session, which created the state's first in-state private equity investment fund. The goal of the $50 million Silver State Opportunity Fund is to capitalize on investment opportunities in Nevada in order to increase funding for Nevada's K-12 schools. A minimum of 70% of investments must be made in businesses located in Nevada, looking to expand in Nevada, or in businesses wishing to relocate in Nevada. Earnings from the fund go to Nevada's K-12 schools."[4]

"In 2013, she worked with Upromise, the Nevada College Savings Plans program manager, to lower the initial account opening cost to a mere $15, making it more affordable for Nevada families to begin saving for their children's future higher education costs. Kate initiated the Ugift and Silver State Matching Grant programs, which provide greater opportunities for family members and friends to contribute to a child's college savings plan. Further, the Distinguished Valor Matching Grant program was established to assist Nevada military families save for their children's college costs." [4]

"In 2013, Kate successfully launched the Nevada College Kick Start Program, which opened SSgA Upromise 529 college savings accounts with an initial deposit of $50 for Nevada public school kindergarten students. No tax dollars were used in establishing these accounts; rather, a portion of program manager fees paid to the Treasurer's Office by our private sector partners provided the necessary funding. The ultimate goal of this program is to create a more "college-going culture" in our state."[4]

On September 18, 2017, Marshall announced her candidacy for Lieutenant Governor of Nevada. She easily won the Democratic primary on June 12, 2018 defeating Lauren Hansen, 67% to 22% and in the November 6, 2018 general election, she easily defeated Republican State Senator Michael Roberson of Henderson: Marshall polled 486,200 votes (50.36%) to Roberson's 421,427 votes (43.65%)